1. Field of the Invention
The present invention relates to the field of energetic, more particularly to generation of photovoltaic direct current followed by its transformation into alternating (AC) or direct (DC) currents. The invention is applicable to photovoltaic power plants and setups connected either to local AC electrical power distribution systems or to energy storage systems, which apply the known from the prior art devices.
2. Description of the Related Art
The drawbacks, attributed to the nature of any photovoltaic cells, are the limiting factors for wide development of plants converting solar radiation directly into electrical energy. These drawbacks arise because photovoltaic cells exist only together with directly connected p-n or hetero-junctions, consequently:                photovoltaic cell generates only DC;        a power generated is directly proportional to the illumination level of the photovoltaic cells surface (insolation).        
Because photoelectric convertors (and photoelectric modules) can generate only direct current (DC), while local industrial electrical power distribution systems (LIEDS) are predominantly AC, a large number of techniques to invert DC into AC have been developed and are known from prior art to date. Nevertheless, all of them are based on the principle of switching the direct current, flowing through the load, ON-and-OFF repeatedly at a fast rate. Devices designed for DC to AC inversion known as DC/AC inverters. In case of photoelectric convertors, the external load is connected in parallel with p-n junction (in the direct direction). This p-n junction absorbs energy of photoelectric convertors when external load is switched OFF, that is one of the most important reasons for losses in the DC/AC inversion.
The directly proportional dependence of current generated upon insolation of the land-based photoelectric convertors is the reason for continuous changes in the initial power of PV modules. This is due to both declining an incidence angle of solar radiation at the surface of solar cells during a day (predictable changes) and changes in the atmosphere transparency (cloudiness—weakly predictable changes). Any deviations of the supplying power from the load power in case of solar cells leads to instantaneous and irrevocable losses of energy.
In order to prevent these losses, more than 25 different methods for tracking the maximum power point of photovoltaic panels (Maximum Power Point Tracking—MPPT) used in the DC/AC inverters have been developed and described to date. Common for these MPPT methods are:                measurement of the actual parameters of the PV module at the exact time;        systematic execution of calculation cycle of PV module power;        systematic adjustments of DC/AC inverter power.        
The main losses of energy generated by PV module are:                losses during the load disconnection required for the systematic measurements of the real parameters of PV module;        losses due to imbalance between the PV module power and the DC/AC inverter power that appears during time between two successive corrections;        switching losses in DC/AC inverter in the short part of each cycle when the device is partially OFF.        
There are several technical solutions to overcome these loses. The examples are:
A circuit arrangement for controlling/regulating photovoltaic systems comprising a plurality of solar generators connected in series and/or in parallel is described in the U.S. Pat. No. 7,709,727 [1], in which each solar generator of the photovoltaic systems is connected with a variable energy bypass that is controlled/regulated in such a manner, that each solar generator is operated continuously in its respective current specific MPP to overcome the losses associated with fluctuation of parameters of the individual PV modules and the difference in insolation conditions at large area.
U.S. Pat. No. 7,456,523 [2] describes a power generation system which comprises a plurality of the connected in parallel sets of PV module and power converters connected to the inverters to supply the alternating current power to a commercial power grid. The system allows to overcome the losses associated with fluctuation of parameters of the individual PV module because each power converter controls the output current and voltage of PV module (maximum power point tracking, MPPT), performs DC/DC conversion to even PV module voltage with voltage of the local industrial grid and DC/AC inversion, synchronization of each power converter with the grid. The described solution provides energy gain by 5-15% and the power generation system continuing function in case of failure or power loss by any of the PV module. The weaknesses of the described administration system are associated to the lack of a compensation of the energy lost due to MPPT control, a high complicity of a system because of necessity to synchronize a plurality of the independent power sources, the use of DC/DC convertor working in the mode ON-OFF.
The U.S. Pat. No. 8,400,134 [3] describes the apparatus and method for tracking the maximum power point (MPPT) of a solar panel, in accordance with which the voltage and current, generated by the solar panel, are monitored and used to generate a pulse signal for charging a capacitor, that decreases the losses associated with ON-OFF mode operation, being typical for both DC/DC and DC/AC converters. When the capacitor voltage exceeds the predetermined level, a part of energy is skipped through the switch causing a short circuit between the current source and the ground. The evident shortcomings and restrictions of this solution are: i) take-off electrical energy from photovoltaic module is realized in impulse mode, similarly as in case of direct connection PV module to DC/DC and DC/AC converters; ii) a part of energy is skipped through a short circuit between the current source and the ground, that is an additional source of energy loss; iii) the apparatus is operating at current of 800 mA, which is a usual limit for electrolytic capacitors.
The apparatus and method, described in the U.S. Pat. No. 7,808,213 and EP Patent 2075895 [4, 5], use a flexible textile capacitor with a total energy from 35 to 112 J. The capacitor is connected to PV module output in parallel; and when the said capacitor is charged to a predetermined charging level, it discharges a current to the charging part of the power supplier of mobile electronic device, e.g. cell phone. Thus the power supplier described operates in a mode like DC/DC converter. At a low insolation level (≤25%) this systems allows to enhance efficiency of 3-7 times as compared with the direct connection of the PV module to the charging part of the power supplier. At an insolation level of about 80%, the gain in energy obtained is 3-6%. The described solution has a quite narrow application, limited by low-power electronic devices and is not provided for exploitation for energy transformation to commercial power grids.
The closest solutions, to what is claimed by this invention, are described in the Ukrainian Patent UA 51651 and Russian Patent RU 2195754 [6,7], which suggest for take-off energy to apply a capacitor charged to the level equal to the MPP voltage of PV module. The apparatus includes a PV module, a capacitor, two threshold voltage controllers adjusted on the upper and lower thresholds, DC/DC converter with pulse-width modulation (PWM), and the feedback means for voltage. Whereas, the upper threshold voltage for triggering off the sensor is set equal to the MPP voltage of PV module at the maximum level (100%) of insolation; and the lower threshold voltage is set to be by 3-5% smaller than the upper threshold voltage. The capacitance has been chosen in the range of 0.02-100 F. The internal resistance of capacitor is chosen to be an order lower than the internal resistance of the PV module under maximum insolation conditions. Power of DC/DC converter is unchangeable in spite of changes in power of the PV module at any certain time. A method of electric energy take-off using the apparatus described provides for the charging of the capacitor to the voltage UB, forming an energy pulse with the power and voltage normalized by the DC/DC converter using energy previously accumulated in the capacitor and energy continually supplied from the PV module. The energy pulses received are used to charge the rechargeable battery (RB).
In accordance to that invention, the method and apparatus provides, due to electric capacity and low internal resistance of the capacitor used, a continuous energy take-off from the PV module at its maximum power point under any operating modes of DC/DC converter, extraction of electrical energy flow in a wide range of PV module power, normalization of the energy impulse over the power and voltage regardless of the instantaneous power of the PV module, more efficient use of energy of PV module—capacitor, reducing energy loss by 25-52% for long-term (daily) exposure of the PV module, simplicity of performance and operation.